This invention relates to a filter for collecting particulates contained in exhaust, and more specifically a filter having increased corrosion resistance and high durability.
Exhausts produced by automobiles are one of the major causers of atmospheric pollution and thus it is very important to develop a technique for removing harmful components from car exhausts.
In case of diesel engine exhausts, it is of utmost importance to remove particulates contained in exhausts, which are mainly NOx and carbon.
In order to remove such harmful contents, various efforts have been made to improve the engine structure. Such efforts include attaching an EGR and improving the fuel injection mechanism. But none of them has been a decisive solution. Another way to remove particulates is to provide a particulate trap in the exhaust passage (Unexamined Japanese Patent Publication 58-51235). This method, which is called the "post-treatment method", is considered the most practical and thus is now under extensive study.
A particulate trap used to collect particulates contained in diesel engine exhausts has to satisfy the following three requirements:
(1) Exhausts collecting capacity
First, such a trap has to be capable of collecting particulates in exhausts to such an extent that the exhausts are purified to a satisfactory level. For this purpose, it is considered necessary to collect at least 60% of the particulates contained in diesel engine exhausts, though it depends on the displacement of the diesel engine and the load on the engine.
(2) Pressure drop
Second, such a trap must not unduly increase the pressure drop of the exhaust. As the particulates collected by the trap increases, the pressure drop when the engine exhausts pass through the trap tends to increase. If the pressure drop exceeds a certain level, a back pressure may act on the engine, which gives a harmful influence on the engine. In order to protect the engine from the harmful influence of the back pressure, it is considered necessary to keep the pressure drop below 30 KPa. For this purpose, the particulates collected by the trap have to be periodically removed to reduce the pressure drop to the original level. If the increase rate of the pressure drop relative to the amount of particulates collected is too high, the particulates have to be removed too frequently to be practical. Thus, it is necessary that such a particulate trap have a low initial pressure drop and that the pressure drop be less likely to increase with an increase in the amount of particulates trapped.
(3) Durability of the material
Third, such a trap has to be made of a material which is durable enough to be capable of withstanding repeated particulates removing treatments for regeneration of the trap. It is considered that the best way to regenerate the trap is to burn off the particulates with an electric heater or a burner. To burn off particulates, the trap has to be heated to a temperature higher than the ignition temperature of the particulates (about 600.degree. C.). Such regeneration treatment has to be done before the back pressure increases to such a level as to lower the engine performance or impair the maneuverability of the vehicle. Each time when a predetermined amount of particulates recollect, the regeneration treatment is repeated. The pressure drop is thus kept under a predetermined level. Thus, the trap has to be made of a material which is so heat-resistant that it can withstand repeated regeneration treatments and which is also so corrosion-resistant that it can withstand corrosive gas contained in exhausts.
Heretofore, a wall-flow type, honeycomb-shaped, porous matter of cordierite ceramic has been considered most desirable as a filter element material that satisfies the above three requirements. But this material has a problem in that particulates tend to concentrate locally. Also, since a cordierite ceramics is low in thermal conductivity, heat spots are likely to develop when regenerating. Thus, there is a possibility that the filter may melt or may crack due to thermal stress. Such a filter is not reliable enough.
To solve this problem, a honeycomb-shaped porous filter made of an SiC ceramic was proposed (Unexamined Japanese Patent Publication 5-23512). Since SiC is high in thermal conductivity, such a filter is less likely to melt or develop heat spots during regeneration. But this material is actually not used for a particulate trap because it requires a prohibitively large calorie for regeneration.
The present applicant proposed a diesel particulate trap which is made of a foamed stainless steel or a composite of such a foamed steel and a metallic unwoven fabric (such as felt) and which can be regenerated without the fear of melting or cracking (Unexamined Japanese Patent Publication 4-86313). But since the stainless steel used is low in resistance to corrosive gas, the life of the trap is unsatisfactory. In particular, an unwoven fabric made of an Fe/Cr/Ni alloy or an Ni/Cr alloy and having a fiber diameter of less than 50 .mu.m tends to suffer a large change in its weight when oxidized in a high-temperature atmosphere of 600.degree. C. or more, though it has a high particulate-collecting capacity. Due to its insufficient corrosion resistance when exposed to diesel engine exhausts, its fibers tend to erode quickly. This leads to deterioration in the particulate-collecting capacity and deformation of the filter. The life of such a trap is thus too short for practical use.
An object of the present invention is to provide a particulate trap which is practical in every sense, and more particularly to provide a metal filter for use in a particulate trap that satisfies all the above-mentioned requirements, that is, high particulate-trapping capacity, low pressure drop, and high durability of the material.